Thursday, October 28, 2010

Gene Study Traces Cattle Herding in Africa

Hanotte and his colleagues analyzed more than a dozen segments of the cattle genome. Because the sections they looked at don't affect how "fit" an animal is evolutionarily, they aren't subject to the effects of natural selection.

As a result, those genetic segments record the genetic twists and turns of different cattle lineages and, in the language of DNA, serve as scribes of bovine history.

The researchers compared this DNA material among many individual cattle belonging to 50 different herds in 23 African nations.

Herders, scientists, and government officials in those countries aided the study by tracking down sometimes-remote herds, testing them, and transmitting the data to Hanotte and his team.

When Hanotte and his colleagues analyzed the samples of cattle DNA, they found that the variation associated with certain segments of genetic code reveal a telling geographic pattern across Africa.

The nature of genetic variation changed like the colors of a rainbow as the researchers looked at cattle from West Africa, Central Africa, and southern Africa. The greatest amount of genetic diversity was found among herds in Central Africa.

Based on the data, Hanotte and his colleagues concluded that people living in Central Africa developed cattle domestication on their own, and that the techniquesor the herders themselvesgradually migrated toward the west and the south, spreading domestication across the continent.

Mixed Origins

In looking at the wide genetic variation among African cattle, the researchers found evidence of interbreeding between cattle native to Africa and an imported breed.

Most modern African herds represent mixtures of two breeds: Africa's native cattle, called taurines (Bos taurus), and a slightly larger Asian breed, known as zebu (Bos indicus), which was domesticated before it arrived in Africa.

Long-distance trade across the Indian Ocean brought many domesticated plants and animals to Africa, including the chicken and camel and cereals such as finger millet and sorghum. Presumably, Hanotte said, trade also brought zebu bulls that farmers interbred with domesticated taurine cows, producing the mixed herds of today.

Some variation in the African herds is also attributable to European influences, Hanotte said. These genetic contributions came in the past few hundred years, during Europe's colonial influence in Africa.

For thousands of years, animal farmers have gradually improved their livestock by selectively breeding animals with different desired traits to endow the offspring with valuable combinations of traits.

Resistance to sleeping sickness is one trait that potentially could spread through selective breeding. Taurine cattle in one region of western Africa, unlike most livestock, are resistant to the parasite that causes the deadly disease.

But the number of animals with the protective adaptation is dwindling, as local farmers give up their taurine herds for large zebu animals.

Hanotte, along with other people, is worried by this trend. "The starting material for selective breeding is diversity," he said. "We can't afford to lose it."